Attenuation of coda waves in the Aswan Reservoir area, Egypt

Tectonophysics 492 (2010) 88–98

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

Attenuation of coda waves in the Aswan Reservoir area, Egypt H.H. Mohamed a, S. Mukhopadhyay b,⁎, J. Sharma c a b c

National Research Institute of Astronomy and Geophysics, Aswan Earthquake Research Center, P.O. Box 152, Aswan, Egypt Department of Earth Sciences, IIT Roorkee, Roorkee-247667, India 70/11-A, Purvawali Ganesh Chowk, Roorkee-247667, India

a r t i c l e

i n f o

Article history: Received 30 October 2009 Received in revised form 18 May 2010 Accepted 19 May 2010 Available online 26 May 2010 Keywords: Aswan Reservoir Egypt Coda Q Lapse time Turbidity

a b s t r a c t Coda attenuation characteristics of Aswan Reservoir area of Egypt were analyzed using data recorded by a local earthquake network operated around the reservoir. 330 waveforms obtained from 28 earthquakes recorded by a network of 13 stations were used for this analysis. Magnitude of these earthquakes varied between 1.4 and 2.5. The maximum epicentral distance and depth of focus of these earthquakes were 45 km and 16 km respectively. Single back-scattering method was used for estimation of coda Q (Qc). The Q0 values (Qc at 1 Hz) vary between 54 and 100 and frequency dependence parameter “n” values vary between 1 and 1.2 for lapse time varying between 15 s and 60 s. It is observed that coda Q (Qc) and related parameters are similar at similar lapse times to those observed for those for Koyna, India, where reservoir induced seismicity is also observed. For both regions these parameters are also similar to those observed for tectonically active regions of the world, although Aswan is located in a moderately active region and Koyna is located in a tectonically stable region. However, Qc does not increase uniformly with increasing lapse time, as is observed for several parts of the world. Converting lapse time to depth/distance it is observed that Qc becomes lower or remains almost constant at around 70 to 90 km and 120 km depth/distance. This indicates presence of more attenuative material at those depth levels or distances compared to their immediate surroundings. It is proposed that this variation indicates presence of fluid filled fractures and/or partial melts at some depths/ distance from the area of study. The Qc values are higher than those obtained for the Gulf of Suez and Al Dabbab region of Egypt at distances greater than 300 km from the study area by other workers. The turbidity decreases with depth in the study area. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Reservoir induced seismicity have been reported from many parts of the world (Gupta, 1992; Talwani, 1997) including areas of low tectonic activity. One such example is that of Aswan Reservoir seismicity in Egypt. Many studies have been carried out on Aswan seismicity and its relation to the reservoir (Kebeasy et al., 1981; Awad, 1994; Awad and Mizoue, 1995; Mekkawi et al., 2004; Awad and Kwiatek, 2005; Haggag et al., 2009). Aswan High dam was built in 1964 on river Nile in Egypt and the resultant reservoir extends ∼ 350 km along the river. It is the second largest reservoir in the world, the first being Lake Volta in Ghana formed in 1965 by the construction of the Akosombo dam. A Mw 5.7 earthquake occurred near the western margin of the reservoir on an E–W trending fault, known as Kalabsha fault, on 14 November 1981 and categorized as a reservoir induced seismicity by Kebeasy et al. (1981). Ever since, earthquakes keep on occurring in this region. Fig. 1a shows location of about 1000 events that occurred during 2004 to 2007 (Haggag et al., 2009). A 13 ⁎ Corresponding author. E-mail addresses: [email protected] (H.H. Mohamed), [email protected] (S. Mukhopadhyay), [email protected] (J. Sharma). 0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2010.05.018

station network was installed here to monitor the earthquakes (Fig. 1b). We have used coda waves of 28 events that occurred during 2007–2009, mostly along the Kalabsha fault and towards east of the 1981 earthquake, to evaluate the attenuation characteristics of the area (Fig. 1b). 2. Geology and tectonics Egypt experiences low to moderate amount of natural seismicity caused by relative motion between African, Arabian and Eurasian plates. Most of this seismicity is confined to its boundary with the Red Sea and is accompanied by high heat flow and swarm type of activity at a few places indicating possible subsurface magma movement (Abdel-Fattah et al., 2008). The study area encompasses a region within 23.25°N to 24°N and 32.3° to 33.05°E, i.e. the northern part of Aswan Reservoir. In this area most of the faults trend either in the E–W or in the N–S direction (Fig. 1). Mostly normal and strike slip faults are present in this area. The most prominent active fault showing right lateral strike slip movement is the E–W trending Kalabsha fault. It is also the longest (∼300 km) fault where the 1981 Aswan earthquake (Mw 5.7) occurred (Kebeasy et al., 1981; Awad, 1994). About 12 km north of the

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Fig. 1. Figure showing the tectonic features, a) seismic network and earthquakes that occurred in the study region during 2004 to 2007 (after Haggag et al., 2009) and b) earthquakes (black dots), whose coda waves were used for this analysis and stations (triangles). The reservoir induced Aswan earthquake of 1981 is shown by a star.

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Kalabsha fault lies the E–W trending Seiyal fault (∼90 km) which is also dominated by right lateral strike slip movement. These two E–W right lateral strike slip faults encompass a graben structure in between them (Awad, 1994; Awad and Kwiatek, 2005). The N–S faults are nearly parallel to the main course of the Aswan Reservoir. Among these, the prominent ones are the Khour El_Ramla fault lying closest to the E–W trend of the reservoir, the Kurkur, Alburqa, Seiyal, Gazalle and Abu Dirwa faults (Fig. 1). The Kurkur fault (∼ 35 km long) and the Khour El-Ramla fault (∼ 65 km long) are dominated by left lateral strike slip motion, and are associated with small folds and steeply dipping beds (Issawi, 1978, 1982). The Gazalle fault (∼35 km long) and the Abu Dirwa fault (∼ 15 km long) are also dominated by left lateral strike slip motion. The area shows generally flat topography with relief varying between 150 and 350 m. Sedimentary rocks of thickness ∼ 500 m overlies granitic/metamorphic basement. The sedimentary rocks consist of the Nubian sandstone, Quaternary calcite and Nile deposits (Issawi, 1982). The igneous and metamorphic rocks are exposed in several localities along the western side of the Reservoir. The Nubian sandstone bed is locally folded in some locations into small anticlines and synclines along the fault traces (Issawi, 1978). Geologically the area is complex with crustal heterogeneity, presence of criss-crossing fault systems, small scale folding of sedimentary rocks and regional uplift. The crustal thickness in Aswan area ranges from 30 km to 35 km (Kebeasy et al., 1992).

3. Data analysis Coda of 28 earthquakes that occurred mostly along the Kalabsha fault was analyzed for estimation of coda Q (Qc) for the study area. These events were recorded by a 13 station network operating in the study area. 3-component broadband sensors (Trillium40 broad band from Nanometrics) were used in the network. Data were recorded digitally at 100 samples/s. The velocity response is flat over the frequency range 0.5 to 50 Hz. The earthquakes occurred within the network with maximum epicentral distance of 45 km and focal depth

of 16 km. The local magnitudes of these events vary between 1.4 and 2.5. The details of these events are given in Table 1. The single back-scattering method of Aki and Chouet (1975) was used for estimation of frequency and lapse time dependence of Qc for the Aswan Reservoir area. Single-back-scattering hypothesis is considered to be a first order approximation of attenuation characteristics of the real earth and is widely used for routine determination of quality factor of coda waves. For this reason we have used it for a preliminary investigation of attenuation characteristics of Aswan Reservoir region. A brief description of the method is given below. Assuming that coda waves are composed of single back-scattered waves from randomly distributed heterogeneities, the coda amplitude can be approximately expressed by the following formula (Aki and Chouet, 1975) Að f ; t Þ = A0 ð f Þt

−1

exp ½−πft = Qc 

ð1Þ

where A( f,t) is the coda amplitude for a central frequency “f” over a narrow bandwidth signal at lapse time t, A0( f) is the coda source factor at frequency f. This is used for the estimation of quality factor Qc of coda waves representing the average attenuation properties of the medium for a given region. Rewriting (1) as ln ½Að f ; t Þt  = ln ½A0 ð f Þ−πft = Qc

ð2Þ

we can calculate Qc by applying a linear regression analysis between ln[A(f,t)t] and time t for each frequency. The Qc values were calculated using SEISAN software package (Havskov and Ottemoller, 2005). Qc was estimated at nine central frequencies (1.5, 2, 3, 4, 6, 8, 12, 16 and 18 Hz) so that its variation with frequency could be studied reliably. All the seismograms were band pass filtered at the chosen central frequencies. An increasing frequency band was used (Table 2) with increasing central frequency to avoid ringing and to take constant relative bandwidths for getting equal amount of energy into each band as suggested by Havskov and Ottemoller (2005) and Rautian and Khalturin (1978). In order to

Table 1 Details of the events whose data are used for the coda Q analysis. Mag and Rms represent magnitude of earthquakes and rms error in seconds respectively. Year

Month

Date

Hour

Minute

Second

Latitude

Longitude

Depth

Mag

Rms

2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2007 2008 2008 2008 2008 2009 2009 2009 2009 2009 2009 2009 2009 2009

3 4 4 4 4 4 4 4 4 4 4 5 7 10 10 7 7 8 10 1 1 1 2 2 2 3 3 3

24 10 12 12 12 12 12 12 12 12 30 16 23 7 15 2 15 11 23 14 24 26 4 8 11 13 18 25

8 10 10 10 13 13 14 14 15 21 9 6 8 11 11 21 15 5 21 12 21 3 8 13 0 5 11 22

18 44 39 58 25 56 17 19 16 27 34 41 23 23 49 40 32 34 28 45 32 14 16 6 30 24 31 29

40.76 12.27 58.37 47.85 35.95 22.94 41.24 22.39 7.12 0.69 31.26 23.6 17.91 31.14 16.76 1.73 1.52 23.07 28.81 48.75 32.52 54.7 27.09 48.9 19.4 35.39 17.45 21.29

18.68 44.20 39.97 58.80 25.60 56.38 17.69 19.37 16.12 27.01 34.52 41.39 23.30 23.52 49.28 40.03 32.03 34.38 28.48 45.81 32.54 14.91 16.45 6.82 30.32 24.59 31.29 29.35

41.07 13.01 59.04 48.83 36.38 23.88 41.53 22.71 7.39 1.14 31.84 24.29 18.30 31.53 17.58 2.40 2.05 23.64 29.28 49.51 33.06 54.95 27.36 49.01 19.91 35.80 17.97 21.78

12 0.1 11.6 9.2 11.4 11.8 10.5 11.5 10.1 8.7 14.4 15.9 3.8 16.3 6.4 2.7 6.5 3.6 0.2 2.3 1.8 5 2.7 2.6 2.5 7 10 2.6

2.1 2 2.1 2.1 2.3 2.3 2.1 2.3 2.3 1.9 2 2.3 2 2.1 2.1 2.3 2.5 2.1 2.4 1.4 2.1 2.3 2.1 2.1 2.2 2.1 2.1 2.2

0.12 0.13 0.26 0.18 0.26 0.26 0.24 0.26 0.29 0.03 0.22 0.23 0.09 0.11 0.19 0.19 0.22 0.16 0.14 0.28 0.22 0.17 0.18 0.17 0.21 0.11 0.2 0.21

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Table 2 Table showing central frequency and bandwidth used for estimation of Qc. Central frequency (Hz)

Bandwidth (Hz)

1.5 2 3 4 6 8 12 16 18

1 1.34 2 2.66 4 5.34 8 10.66 12

study the lapse time dependence of Qc, coda windows of length W of 20 s were selected at lapse times tstart (i.e. the time of the coda window after origin time) of 15 to 60 s with an interval of 5 s. The lapse times were chosen such that they were always greater than twice the S-wave travel time ts to avoid the data of the direct S waves (Rautian and Khalturin, 1978; Havskov and Ottemoller, 2005). Fig. 2 shows an example of a seismogram whose data were used for this analysis. The filtered seismic traces within these coda windows are smoothed by calculating root mean square (rms) values of coda amplitudes of the filtered seismograms with a sliding window of length equal to 5 cycles of each central frequency. The smoothing window is slid along the coda in steps of half the window length and the value at each step in the same frequency band is evaluated. These rms values constitute a smoother envelope of the coda. The rms amplitude of the last 5 cycle length of lapse time window is divided by noise data of the same length before the onset of the P wave to calculate the signal to noise (S/N) ratio. The calculated Qc values at given central frequencies were accepted only when correlation coefficients (C) for the best-fit line for the coda decay slope with respect to lapse time were greater than 0.5 and signal to noise ratio (S/N) was greater than 2 for a given data set. The Qc values for such data set were averaged for each central frequency. Total number of waveforms used for this analysis for each lapse time is shown in Fig. 3. The results are discussed in the next section. 4. Results and discussion Fig. 4 shows the plot of Qc versus frequency for different lapse times. The frequency relation parameter for Qc can be represented as Qc = (Q0 ± ΔQ0)f(n ± Δn), where Q0 is Qc at 1 Hz frequency, n is the frequency relation parameter and ΔQ0 and Δn are the respective errors. The (Q0 ± ΔQ0) and (n ± Δn) values are shown in the plots for all the lapse times and given in Tables 3a and 3b. In order to check how Qc varies with lapse time Qc versus average lapse time (tc) values are plotted in Fig. 5, where tc = tstart + W/2, where W is the window

Fig. 3. Plot of number of observations at a given lapse time.

length. The minimum and maximum volumes sampled by the coda depend on tstart and tstart + W respectively. The average volume sampled can be assumed to be represented by the average lapse time tc (Havskov et al., 1989). A plot of Qc versus tc gives an idea about how the expanding volume of material from which coda reaches a station affects Qc. For larger lapse times larger volumes are sampled by the coda waves. Larger volume incorporates effect of deeper regions. Hence these variations in Qc reflect the variation of coda attenuation from larger volume/deeper zones of the study area. It is observed from Fig. 5 that Qc values in general increase with increasing lapse time. However, the rate of increase is not uniform. At some frequencies and lapse times the Qc values decrease with increasing lapse time or remain almost same. Such discrepancy is observed around 45 s and 65 s average lapse times for most of the frequencies. It has been observed by numerous seismologists that Qc varies with lapse time and generally increases with increasing lapse time (Aki and Chouet, 1975; Sato, 1977, 1978; Roecker et al., 1982; Pulli, 1984; Rhea, 1984; Jin and Aki, 1986; Phillips and Aki, 1986; Gagnepain-Beyneix, 1987; Havskov et al., 1989; Kvamme and Havskov, 1989; Del Pezzo et al., 1990, 1995; Ibañez et al., 1990; Nishigami et al., 1990; Akamatsu, 1991; Kanao and Ito, 1991; Kosuga, 1991; Del Pezzo and Patané, 1992; Hellweg et al., 1992; Giampiccolo et al., 2002, 2004; Tuvè et al., 2006; Mukhopadhyay and Tyagi, 2007; Mukhopadhyay et al., 2008). They think that a possible explanation of the observed lapse time dependence of Qc could be attributed to depth-dependent variation in this parameter. Depth-dependent variation in attenuation is indicative of variation in level of heterogeneity with depth. Wennerberg (1993) showed that this trend cannot be explained by multiple scattering model of Zeng (1991), but requires variation in intrinsic and/or scattering

Fig. 2. Vertical component seismogram showing record of an earthquake that occurred on 16 May 2007 at 6 h 41 min and 23.6 s, located at 23.541°N, 32.616°E and 15.9 km depth and recorded at AHD. Maximum amplitude is 180491 in counts. Origin time (OT) and P- and S-arrival times are shown with vertical arrows. Coda window for a lapse time of 30 s and window length 20 s is also shown with a box.

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Fig. 4. Qc versus frequency plots for lapse time tstart of a) 15 s, b) 20 s, c) 25 s, d) 30 s, e) 35 s, f) 40 s, g) 45 s, h) 50 s, i) 55 s and j) 60 s. The Qc frequency relations and standard deviations are also shown in each plot.

attenuation within the expanding volume involved in the coda generation process. Abubakirov and Gusev (1990) and Hoshiba (1991) too suggested the same. According to Roecker et al. (1982), Kvamme and Havskov (1989), Ibañez et al. (1990), Wennerberg

(1993), Del Pezzo et al. (1995), Woodgold (1994) and Akinci et al. (1994), the plausible cause of increase in Qc with lapse time is decrease in intrinsic attenuation with depth. On the basis of special tests Del Pezzo et al. (1990) came to the conclusion that the observed

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93

Fig. 4 (continued).

lapse time dependence of Qc can be explained by depth-dependent variation in attenuation, although some effect is also due to inappropriate assumptions in the single back-scattering model. The non-uniform variation of Qc with lapse time for our case indicates that in the study area, Qc does not increase uniformly with depth and may even be lower at certain depths compared to surrounding materials. According to Woodgold (1994), the variation in Qc with lapse time can be caused by any of several other factors such as (i) consideration of non-zero source–receiver distance with non-isotropic scattering, Table 3a Table shows the Q0 and n values with standard deviations for different lapse times. Lapse time to beginning of window(s)

Average lapse time tc (s)

Average depth “h” (km)

Q0

±ΔQ0

n value

± Δn

15 20 25 30 35 40 45 50 55 60

25 30 35 40 45 50 55 60 65 70

50 58 67 77 86 95 104 113 122 129

54 54 64 70 68 73 86 95 82 100

6 3 3 1 4 3 2 7 2 4

1.16 1.2 1.14 1.06 1.05 1.09 1.05 1.02 1.07 1

0.07 0.03 0.02 0.01 0.04 0.02 0.01 0.03 0.01 0.02

(ii) use of a 2-D model instead of a 3-D model, (iii) use of a single scattering model where multiple scattering is significant. When lapse time is much larger than the S-wave travel time, the effect of the first assumption is insignificant. Besides it is observed that when medium is considered isotropic (Aki and Chouet, 1975; Pulli, 1984; Del Pezzo and Patané, 1992; Hellweg et al., 1992; Akinci et al., 1994; Del Pezzo et al., 1995; Gusev, 1995; Akinci and Eydo˘gan, 1996; Baskoutas, 1996; Giampiccolo et al., 2002, 2004; Tuvè et al., 2006; Mukhopadhyay and Tyagi, 2007; Mukhopadhyay et al., 2008) Qc shows lapse time dependence. However, even when non-isotropic scattering is assumed (Abubakirov and Gusev, 1990; Gusev, 1995) Qc shows lapse time dependence. Thus it is generally accepted that lapse time dependence of Qc represents variation of attenuation in the medium. Kopnichev (1977) and Gao et al. (1983) demonstrated that the effect

Table 3b Q0 and n values for Koyna region (after Gupta et al., 1998). Lapse time (s)

Q0

n

20 30 40 50 60

66 96 131 148 182

1.16 1.09 1.04 1.04 1.02

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Fig. 5. a) to j) Qc versus average lapse time/depth plots for frequencies of 1, 1.5, 2, 3, 4, 6, 8, 12, 16 and 18 Hz frequencies. k) Plot of n value versus average lapse time/depth. The standard deviations are also shown in each plot.

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Fig. 5 (continued).

95

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of multiple scattering becomes insignificant for local events with a smaller lapse time. However, the exact value of lapse time for which multiple scattering becomes significant depends on medium heterogeneity and level of intrinsic attenuation. Sato (1988) showed that even for large lapse times single scattering dominates over multiple scattering when the fractal dimension of number of scatterers in a given region is less than 2. The Q0 (i.e. Qc at 1 Hz) and n values (Tables 3a and 3b) for Aswan Reservoir are similar to those observed for Koyna reservoir in India at similar lapse times (Gupta et al., 1998). However, the variation of Q0 and n with depth for our study region is not linear. At certain average lapse time ranges Q0 remains more or less constant or becomes lower than adjacent values (Fig. 5a). Such variations are also observed at other frequencies, as discussed in the next section. This could be indicative of presence of partial melts and/or fluid filled fractures at these depth levels. The n values vary within a range of 1 to 1.2 (Fig. 5k). Hence, its variation is not very strong. The average n value is around 1.1, which is similar to that observed for the Koyna reservoir area in India (Gupta et al., 1998) and also to those observed in tectonically active and complex regions. The reason for similarity of attenuation parameters for reservoir regions and tectonically active regions is not understood. 4.1. Variation of Qc with depth Sato (1978) and Pulli (1984) have shown that the scatterers responsible for the generation of coda waves are generally assumed to be distributed over the surface area of an ellipsoid which can be calculated using the following formula: x2 y2 ! =1
2 +
 2 vtc vtc R2 − 2 2 4

ð3Þ

where x and y are the surface co-ordinates, R is the hypocentral distance, v is the velocity of S-wave and tc is the average lapse time, respectively. We have used S-wave velocity of 3.5 km/s and average lapse time duration tc = tstart + W/2. The average depth of volume of medium from which coda wave generation would occur for different lapse times is given by the formula h = √{(vtc/2)2 − (R/2)2} + hav, where hav is the average hypocentral depth (Pulli, 1984; Havskov et al., 1989; Canas et al., 1995; Mukhopadhyay et al., 2008; Rahimi and Hamzehloo, 2008). The values of ‘h’ for different tstart values are given in Tables 3a and 3b. This could be considered as the average depth of investigation related to a particular coda lapse time. However, it should be remembered that this depth estimates are approximates as the velocity in the real earth varies with depth. The average depth of investigation increases with increasing average lapse time. The average depth of investigation “h” versus Qc for different frequencies are plotted in Fig. 5 to understand how attenuation of coda of different frequencies varies with average depth. It is observed that at all frequencies Qc does not increase systematically with depth/lapse time as is the case for Koyna reservoir region or many other regions of the world. Between ∼ 70 km and ∼90 km Qc remains more or less constant at frequencies lower than about 4 Hz and at higher frequencies it becomes lower than those at surrounding depths. Below 100 km average depth Qc becomes more or less constant at frequencies higher than 8 Hz. At lower frequencies Qc is lower at ∼ 120 km average depth than that at surrounding depths. The reason for such variations could be that the media at ∼ 70 km to ∼90 km and ∼120 km average depths are more attenuative compared to those immediately above and below them. Although Haggag et al. (2009) carried out travel-time tomography for this region using local earthquake data; they could reveal 3-D variation in P- and S-wave velocity (Vp and Vs respectively) and Vp/Vs structure only for the top 20 km. Presence

of fluid filled fractures at those depths could be a possible reason for such variation in attenuation characteristics with lapse time/depth. However, at such depths it does not seem possible that fluid filled fractures would be due to the presence of the reservoir. Alternatively, lateral variation in attenuation properties or presence of partially molten material at some depth could also explain such variation. Presence of molten rocks in Abu Dabbab, ∼ 300 km NE of the present study area was proposed by Abdel-Fattah et al. (2008). This could also explain the observed variation in Qc with lapse time/depth in the Aswan Reservoir area, especially presence of partially molten regions at some depths could be possible. However, they obtained very low values of Qc for their study area as mentioned above, which is much lower than those obtained by us. Although most workers interpret variation of Qc with lapse time in terms of its variation with depth, one has to remember that with increasing lapse time coda from a larger volume of material reach a station. This could mean that coda from regions at a larger distance laterally would also affect our observation. Hence, an alternate explanation for above observation could be presence of more attenuative medium at around 70 to 90 km and around 120 km distance from the region of study causes such variation of Qc with lapse time. At present, we do not have any information about crustal structure of regions at such distances from our network that can be compared with our results. 4.2. Comparison of result with global observations We compare the Q0 and n values of our study region with that of Koyna region in India, where reservoir induced seismicity is well documented (Gupta et al., 1998). Gupta et al. (1998) obtained coda Q for 13 earthquakes of magnitude b3.0 occurring within epicentral distance b60 km and focal depth b10 km. As these parameters are similar to those for our data set, we make a comparison of the Q0 and n values of Koyna region (Table 3b) with that of ours (Table 3a). We observe that the Q0 values for Koyna region are slightly higher than that of Aswan region. This could be because Koyna is located in a tectonically stable region, whereas Aswan is located in a moderately seismically active area. It is also noted that Q0 values increases systematically for Koyna region but does not increase systematically for Aswan region. Abdel-Fattah et al. (2008) analyzed coda of microearthquakes of Abu Dabbab region of eastern Sahara in Egypt, which lies approximately 300 km NE of our study region. They obtained frequency relations for that region as follows: Qc = (9± 1)f(1.1 ± 0.03), Qc = (16 ± 1)f(1.0 ± 0.03), Qc = (22 ± 1)f(0.9 ± 0.03) and Qc = (29 ± 1)f(0.9 ± 0.04) at 10, 20, 30, and 40 s, respectively. It is observed that the n values are comparable to those observed for Aswan Reservoir region, but the Q0 values are very low compared to those obtained by us. However, it should be noted that such low Q0 values are not observed for the lapse times mentioned above for any other region of the world. However, it is to be noted that Morsy and El Hefnawy (2004) obtained Q0 value varying between 18 and 62 over an ∼150 × 200 km2 area around the Gulf of Suez which is ∼300 to 500 km NE of our study area. This area encompasses the Abu Dabbab region and the values of Q0 given in the two studies are comparable. Our Q0 values are similar to those obtained for reservoir induced seismicity of Koyna region (Gupta et al., 1998) and those for tectonically active regions as mentioned before, whereas those for Abu Dabbab region are similar to those for volcanic regions. In the Abu Dabbab region presence of magma is proposed based on heat flow data and swarm like occurrence of microearthquakes and very low value of Q0 as reported before (Abdel-Fattah et al., 2008). A method of comparing Qc values from different regions of the world was proposed by (Gusev, 1995). He plotted Qc versus lapse time for central frequency 1.5 Hz, as well as frequency relation coefficient n versus lapse time for world wide data. He derived formulation for Qc and n variation with lapse time for an earth model where a turbid crust

H.H. Mohamed et al. / Tectonophysics 492 (2010) 88–98

overlies a transparent mantle. The turbidity in the crust was assumed to decrease with depth by him. Turbidity represents randomness of a heterogeneous medium (Sato and Fehler, 1998). He considered cases where intrinsic attenuation were either zero or had some fixed value for a given frequency for the entire crust. According to his theoretical model when intrinsic attenuation is zero then Qc =

2πftc N

ð4Þ

and when it is not zero then 1 N 1 = + Qc 2πftc Qi

97

(1995). Qc versus lapse time for the present data set shows good match with the theoretically predicted values of Gusev (1995). Similar results were also obtained by Mukhopadhyay et al. (2008) for Chamoli region in Garwhal Himalayas, India. This shows that in the Aswan Reservoir area turbidity decays fast, i.e. its decay is proportional to between 2nd and 3rd power of depth. On the other hand, the frequency relation coefficient ‘n’ in the relation Qc = Q0fn are systematically too high compared to values theoretically estimated by Gusev (1995). As proposed by Gusev (1995) from theoretical modelling, this fact also indicates that turbidity decays rapidly with depth in the study region.

ð5Þ

where N is the power in the power-law decay of turbidity with depth, f is the frequency and Qi is the intrinsic Q. Gusev (1995) plotted the resultant theoretical curves along with the world wide data. We have used his plots as base and superimposed the results of our analysis on them. In Fig. 6a and b variation of Qc with lapse time for central frequency of 1.5 Hz and n versus lapse time for Aswan Reservoir area (plus sign) is superimposed on that for various other regions of the world. Average lapse time is taken to be tstart + W/2, as specified by Gusev

5. Conclusions The estimated Qc values for the Aswan Reservoir area show nonlinear increase with lapse time. At some lapse times they decrease or remain more or less constant with increasing lapse time. This might indicate that Qc does not increase uniformly with depth, as is observed in most other places. However, with increasing lapse time the volume of material from coda waves reach different stations also increases. The lateral extent of this volume also increases. This means that the observed lapse time variation could also be due to lateral changes in the crust. Based on the available results it is concluded that materials at 70 to 90 km and around 120 km depth/distance from the study area are more attenuative. This may indicate presence of fluid filled fractures and/or partially molten material at those places. A comparison of Q0 and n values for the study area and that for Koyna reservoir region in India shows that Q0 values for Aswan Reservoir region for different lapse times are slightly lower than those for the Koyna reservoir region. This could be because Aswan lies in a moderately seismically active region, whereas Koyna lies in a tectonically stable region. The n values for both the region are comparable. It is interesting to note that Q0 and n values for both the regions are similar to those for tectonically active regions of the world, although the reason for such similarity is not understood. Compared to the more seismically active Al Dabbab region of Egypt Qc values in Aswan region are higher. Qc versus lapse time plot for Aswan region shows good match with the theoretically predicted values of Gusev (1995) but n value versus lapse time plots are systematically higher than his theoretical predictions. This shows that turbidity decreases with depth in the study region.

Acknowledgements We are thankful to the Director of NRIAG and the staff of Aswan network for their kind help and support. We are also thankful to Prof. J.R. Kayal for his help and support. Constructive suggestions by two unknown reviewers and Prof. Hans Thybo, Editor-in-Chief, helped greatly in improving the article.

References

Fig. 6. Plot of a) Qc versus average lapse time for 1.5 Hz frequency and b) n value versus frequency for Aswan Reservoir region superposed on that for a number of regions around the world. Data for all other regions are from Gusev (1995). Theoretical values for these parameters obtained by Gusev (1995) are shown by dashed lines.

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